42 ORGANS OF SECRETION. 



Organs of Secretion. 



We may consider under this head the kidney, mucus gland, 

 purple gland and anal gland : all special organs situated in the 

 respiratory cavity or opening into it. 



Tlic kidney is a large, hollow, glandular mass at the base of 

 the respiratory cavity, close to or sometimes perforated (Triton) 

 by the rectum. It contains a fluid having a whitish or brownish 

 appearance, tilled with hard granules, and in which Jacobson 

 first detected the presence of uric acid, ammoniac and salts of 

 lime. 



If the kidney is cut open the internal cavity is observed, which 

 is, however, much narrowed by numerous thick, spongy, crimped 

 annular folds or meshes, which clothe it internally. The sponge- 

 walls, the surface of which, because of the folds is much increased, 

 are covered with round cells, which excrete the urinary products. 

 At the wall of the cells, at least in the youngest ones, there is 

 alwa} r s a distinct nucleus and its contents consist sometimes of 

 a yellowish or greenish fluid, and within are concentrically lami- 

 nated urinary concretions. By bursting, these secretory cells 

 allow the urinary concretions to escape into the water in the 

 kidney. 



Externally, the kidney is spun over by a very thick and strong- 

 vascular network of the venous system. This vascular network 

 has several openings into the kidney, through which the urinary 

 products are mingled with the blood ; and, consequently, when 

 these are examined microscopically, they are found to contain, 

 besides renal cells and free concretions, a considerable number 

 of blood corpuscles. 



Usually the kidney opens into the base of the respiratory 

 cavity by means of a transverse, slit-like opening, encircled by a 

 strong sphincter muscle (Triton. Dolium, Cassis, Murex, Lit- 

 torina, Natica, etc.) ; sometimes, however, it has a gut-like, 

 efferent duct, a ureter, which passes forward between the rectum 

 and sexual canal, opening outwards not far behind the anus 

 (Paludina, Turbo, Voluta, Conns). There are accordingly three 

 efferent canals alongside of each other at the right side of the 

 respiratory cavity, viz., the rectum, ureter, vagina or vas deferens. 

 The ureter is usually a somewhat dilated canal and is not simply 



